1. Field of the Invention
The present invention relates to a toner support suitably used as a developing roller which is provided, in an electrophotographic apparatus or electrostatic recording apparatus such as a copying machine or printer, for visualizing an electrostatic latent image by using a non-magnetic one-component developer, and an image forming apparatus such as a developing apparatus including the toner support. In particular, the present invention relates to a toner support for preventing occurrence of fogging on a white image, thereby forming a high quality image without occurrence of a change in image with time and uneven image even upon continuous printing or partial printing of a sold image, and an image forming apparatus including the toner support.
2. Prior Art
According to a prior art printing process using an electrophotographic image forming apparatus such as a copying machine or printer, development has been performed by supplying a non-magnetic one-component developer to a photosensitive drum on which a latent image is retained, to allow the developer to adhere on the latent image formed on the photosensitive drum, thereby visualizing the latent image. As such a developing method, a press-developing method has been known, for example, from U.S. Pat. Nos. 3,152,012 and 3,731,146. According to this method, since the developer does not require a magnetic material as a carrier, it is easy to simplify and miniaturize the apparatus and since the developer does not contain a magnetic powder, it is possible to keep up with formation of a color image.
In this press-developing method, development is performed by bringing a developing roller (toner support), on which a toner (non-magnetic one-component developer) is supported, into contact with a latent image retainer (image forming body) such as a photosensitive drum, on which an electrostatic latent image is retained, to allow the toner to adhere on the latent image formed on the latent image retainer, and accordingly, the developing roller is required to be formed of a conductive elastic body.
The press-developing method will be more concretely described with reference to FIG. 4. Referring to this figure, a developing roller (toner support) 1 is disposed between a toner-coating roller 4 for supplying a toner 6 and a photosensitive drum (image forming body) 5 for retaining an electrostatic latent image. The toner 6 is supplied from the toner-coating layer 4 to the surface of the developing roller 1 by rotating the developing roller 1, photosensitive drum 5, and toner-coating layer 4 in the direction shown by arrows in FIG. 4. The toner 6 thus supplied onto the developing roller 1 is formed into a thin layer having a uniform thickness by a layer forming blade 7. Then, by rotating the developing roller 1 in contact with the photosensitive drum 5, the toner 6 formed into the thin layer on the developing roller 1 adheres on the latent image formed on the photosensitive drum 5, to thereby visualize the latent image. In the figure, reference numeral 8 designates a transfer unit at which a toner image is transferred on a recording medium such as a paper sheet, and 9 is a cleaning unit at which the toner remaining on the surface of the photosensitive drum 5 after the transfer step is removed with a cleaning blade 10.
In the above-described developing process, the developing roller 1 must be rotated while certainly holding the state being in close contact with the photosensitive drum 5. To meet such a requirement, as shown in FIG. 1, the developing roller 1 has a structure in which a semi-conductive elastic layer 3 is formed around the outer periphery of a shaft 2 made from a metal having a good conductivity. The semi-conductive elastic layer is formed of a semi-conductive elastic body made from an elastomer such as silicone rubber, NBR, EPDM, ECO, or polyurethane to which carbon black or a metal powder is dispersed or a foamed body obtained by foaming the elastomer. In some cases, a covering layer 3a made from a resin or the like is formed on the surface of the semi-conductive elastic layer 3 for controlling the charging and adhesion characteristics to the toner, controlling a friction force between the developing roller and the layer forming blade, and preventing the photosensitive drum 5 from being contaminated by the elastic body forming the developing roller.
A method of forming an image by allowing a toner supported on the toner support to directly fly on a paper sheet or OHP sheet via a hole-shaped control electrode has been also proposed.
To obtain an electric field required for transferring the toner supported on the toner support to the image forming body, the resistance of the toner support is adjusted at a value of about 10.sup.5 to 10.sup.9 .OMEGA.. In many cases, to easily adjust the resistance of the toner support, the resistance of the resin-covering layer 3a is set at a value higher than that of the semi-conductive elastic layer 3. Like the adjustment of the resistance of the semi-conductive elastic layer 3, the adjustment of the resistance of the resin-covering layer 3a is often performed by adding carbon black, a metal powder, a metal oxide, and the like thereto.
To obtain a high performance, particularly, a high quality image in a printing process using an electrophotographic system including the above-described developing roller (toner support), the developer supported on the developing roller is required to be usually in a uniform charged state while keeping constant values of charges until the developer is transferred onto the image forming body.
If the electric characteristics of the developing roller vary over the entire surface of the roller, there arise the following problems. Namely, in the case of forming a high quality image or forming a color image by using an image forming apparatus such as a printer, there occur an image failure such as stain, uneven image density, and fogging on a white image. Upon continuous printing, the charged amount of the developer supported on the developing roller is often made unstable and gradually changed, with a result that the toner at a non-developed portion on the developing roller is continuously charged by friction, thereby causing an inconvenience that the charged amount exceeds a specific value. Upon partial printing of a black solid image, the charged amount of the developer newly supported on a portion at which the black solid image has been printed is different from that at the periphery thereof, with a result that there occurs an uneven image due to unevenness of the developer charging distribution. Such phenomena tend to occur under the condition that the charges imparted on the surface of the toner support are less escaped therefrom.
In this case, charges imparted on the surface of the toner support may be grounded from the surface mainly via the conductive shaft, to be thus decayed. From this viewpoint, for a developing sleeve used for development by a magnetic toner, since the metal base is very conductive, the surface charges are allowed to easily flow to a ground portion, and therefore, there is no problem associated with the residual surface charges; however, for the toner support mainly made from a semi-conductive material used for development by a non-magnetic one-component toner, since the resistance of the semi-conductive material is high, the residual charges are less escaped and thereby remain on the surface of the toner support for a long time, with a result that there arise inconveniences caused by the residual charges.